Among properties sought-after in a lubricating oil, the performance characteristics of (1) high oxidative stability, (2) satisfactory fluidity at low temperatures, (3) high viscosity index and (4) satisfactory lubricity in load-carrying capacity, anti-wear capacity, etc., can be given.
Generally, mineral oils have been employed as oils in lubricants. In recent years, however, from an environmental preservation standpoint there have been calls centered in Europe for highly biodegradable oils, and the development of a base oil which replaces the poorly biodegradable mineral oils has been desired.
As highly biodegradable base oils there are vegetable oils (the biodegradation rate according to the CEC-L33-A-93 method is 90-100%) and polyolesters (ditto, 55-100%). Vegetable fatty oils possess the drawback of inferior oxidative stability. Nevertheless, vegetable fatty oils have advantages such as high viscosity indices, excellent lubricity in extreme pressure, low volatility and good compatibility with additives.
Accordingly, in recent years studies on improving the use of vegetable fatty oils in lubricant base oils have been made. For example, Pat. Laid-Open 209187, 1993 discloses a technology of improved cold resistance, i.e., low-temperature fluidity, by adding esters of polyglycerine fatty acid, esters of sucrose fatty acid, and lecithin to a liquid oil such as rapeseed oil. However, in the above-noted technology, because a liquid fat such as rapeseed oil is used, its oxidative stability is poor.
Further, Laid-Open Pats. 14710, 1994 and 179882, 1995 disclose a technology of improved low-temperature fluidity that introduces a medium-chain saturated fatty acid into a liquid fat such as rapeseed oil by ester interchange. However, due to the use of a fatty oil such as rapeseed oil that contains a large quantity of unsaturated fatty acid, the oxidative stability is not good (e.g., in Laid-Open Pat. 179882, 1995, the Ransmatt stability is in the range of 4 to 5 hours).
Meanwhile, there is also a technology that introduces a medium-chain saturated fatty acid into a hydrogenated coconut oil, palm kernel oil, etc. fatty oil whose unsaturated fatty acid content is scant (Laid-Open Pat. 314790, 1992). Nevertheless, although oxidative stability is improved with this technology, it turns out not to be satisfactory because solid fat is utilized and the cloud point is high (0.degree. C. or more in the majority of instances). Therefore, in order to lower the cloud point, a large amount of medium-chain triglycerides (MCTS), which are expensive, is necessary.
Fatty oils which become solid at ordinary temperatures, such as the foregoing palm oil, beef tallow and hardened oil, that are to be raw materials are of comparatively good oxidative stability, but since their melting points are high, their low-temperature fluidity is poor. Meanwhile, wherein a fatty oil such as linseed oil and fish oil which contains many highly unsaturated fatty aids, or a fatty oil such as rapeseed oil and soybean oil which contains much linoelic acid, is utilized independently as a lubricating oil, it will be of comparatively good low-temperature fluidity, but the oxidative stability will be poor. As a fatty oil endowed concurrently with oxidative stability and low-temperature fluidity, medium-chain saturated fatty acid triglycerides can be given. However, wherein they are utilized independently, lubricity deteriorates, since compared with general vegetable oils (palm oil, rapeseed oil, etc.) the alkyl group is a short chain.
In other words, a lubricating oil in which vegetable oil is made the base, and which is a base oil concurrently endowed with oxidative stability and low-temperature fluidity, at present has not yet been sufficiently developed.
The object of the present invention is to develop a biodegradable lubricant base oil of good fluidity at low temperatures, of low cloud point, and furthermore of good oxidative stability and lubricity.